Each node (category, forum etc) has a display order in its settings. Set the display order so that the category you want to appear at the top has the lowest display order. You can then do the same with forums that appear within each category (i.e. they are a child of that category).
e.g.
Category1 - 10
Forum1 - 10
Forum2 - 20
Forum3 - 30
Category2 - 20
Forum4 - 10
Forum5 - 20
Forum6 - 30
Category3 - 30
Forum7 - 10
Forum8 - 20
Forum9 - 30